1. Field of the Invention
The present invention relates to the handling of distortion introduced by semiconductor devices. More specifically, the techniques of the present invention provide mechanisms for compensating for memory effect in lookup-table-based digital predistorters.
2. Description of the Prior Art
In processing a signal through a semiconductor device, it is advantageous to be able to predict with some regularity the outcome of the processed signal. Often, however, the design of the semiconductor device produces an outcome that does not conform ideally to the function of the semiconductor device. This is because in some cases the semiconductor device can introduce distortion into the processed signal.
For instance, in wireless systems, a power amplifier is often used to drive an antenna. Ideally, the power amplifier produces output signals that have a linear relationship with the input signals. However, power amplifiers typically produce output signals that deviate from this linear relationship to some degree. As such, decoding the signal at the receiver side becomes more difficult and may result in interference in other frequency bands. The amount of deviation can be viewed as the distortion introduced by the power amplifier.
In an attempt to control the distortion introduced by the power amplifier, digital predistorters may be used. Generally, a digital predistorter applies to the input signal a correction factor that is the inverse of the distortion amount. Some predistorters implement a lookup table for storing and applying the correction factor. See Experimental Performance of an Adaptive Digital Linearized Power Amplifier, Andrew S. Wright, Willem G. Durtler, IEEE Transactions on Vehicular Technology, Vol. 41, No. 4, November 1992, the contents of which are hereby incorporated by reference. Although the lookup-table-based predistorter provides a level of distortion compensation to the output signal, the output signal can still deviate from the linear relationship. This is because some lookup-table-based predistorters are designed on the premise that the distortion only depends on the current input. In practice, it has been found that the distortion also depends on the previous inputs. This condition is commonly referred to as the “memory effect” in power amplifiers. The memory effect can fundamentally limit the effectiveness of distortion compensation mechanisms such as lookup-table-based predistorters.
Some lookup-table-based predistorters do compensate for the memory effect. However, their methods of implementation are typically complex or expensive, for example, the use of lookup table values derived from an internal representation of measured Volterra coefficients. Some predistorters are not lookup-table-based, but do compensate for the memory effect of the amplifier. However, these predistorters also implement methods that are complex to use and less efficient as compared to lookup-table-based predistorters.
It is therefore desirable to provide improved methods and apparatus for compensating for the memory effect of semiconductor devices. More specifically, it is desirable to provide techniques and mechanisms for efficiently and effectively compensating for the memory effect in lookup-table-based digital predistorters.